I use Cinema4D R11 in combination with MeshLab to check for holes and invert face normals. C4D seems to have a bug in its STL export plugin. It always exports STL with inverted normals. But that's not the problem. If it had been, Shapeways would have sent me another message.

An upload in WRL format by the way results in the same message. I haven't tried any other format so far.

In C4D my default unit is millimeter, so I think I have to choose the same unit in the Shapeways upload dialog. I don't want to get an object that is 25.4 times the size that it's actually supposed to have.